As the desire for more intensive electronic applications increases, so does the demand for electrical systems that operate at faster speeds, occupy less space, and provide more functionality. To meet these demands, manufacturers design modules containing numerous components with different package types, such as integrated circuits (ICs), multi-chip modules (MCMs), hybrids, and the like, residing in relatively close proximity on a common substrate, for example, a circuit board. Certain components residing on the circuit board, such as a central processing unit (CPU) or processor, generate large amounts of heat which must be dissipated by some means.
Generally, heat is dissipated by transferring the heat to a heat-sinking medium such as air or water. Due to the expense and complexity associated with liquid media and, in many cases, the non-availability of such media, it is desirable to use air as a sinking medium. Heat-transfer from the heat source to the surrounding air is accomplished via direct contact between a component and the surrounding atmosphere, passive thermal transfer schemes (e.g., heat pipes), or active liquid cooling systems (e.g., a closed loop circulating cooling system) or a combination of these schemes. In the case of direct contact, heat transfer is generally enhanced by placing a thermally conductive heat sink with protruding fins in contact with an area of high heat flux, such as the upper surface of a component's package or the component's "face." The heat sink fins greatly increase the heat transfer area to the surrounding atmosphere and reduce the thermal resistance between the heat source and heat sink. Typically, the surrounding air circulates over the heat sink fins by convection; however, in order to further enhance the heat transfer to the surrounding atmosphere, a fan may be used to mechanically move air over the heat sink fins.
In order to enhance the transfer of heat within the heat sink itself, some heatsinks enclose a heat pipe, while others attach to a separate housing which encloses a heat pipe. Such an enclosed heat pipe provides a thermally efficient conduit for transferring heat from small areas of high heat generation uniformly throughout the heat sink, creating a nearly isothermal surface on the heat sink.
In the prior art, an individual heat sink is typically adhesively bonded to (e.g., with a thermosetting, conducting epoxy) and/or mounted adjacent the face of a single heat-generating component with fastening devices (e.g., clips, retaining rings, press fits, and the like). For circuit boards having a reasonable number of components, with ample component-to-component spacing, the prior art use of individual heatsinks and fastening devices is usually effective for transferring heat away from the critical components of a circuit board.
As the complexity of a circuit board increases, however, the number and type of components are likely to increase, while the allotted space between components is likely to decrease. These two factors result in a densely populated, complex circuit board. These boards also have a multilevel surface due to the various heights of the numerous components, surface anomalies and fabrication tolerances such as inconsistencies resulting from solder ball attachments. Since many, and possibly even all, of the components on a circuit board require cooling, the high component density and multi-level surface coupled with the requirement that each heat sink be in intimate contact with its associated component results in a board with numerous individual closely spaced, multilevel heatsinks.
Furthermore, for dissipating heat generated by high power components, such as the next generation, SUN UltraSPARC.RTM. family of processors (in particular, those used in the next generation workgroup server), the size of a heat sink must be relatively large, often ten times the size and weight of the actual component to which it is attached. (UltraSPARC is a registered trademark of SPARC International, Inc. and is licensed by Sun Microsystems, Inc.) This size requirement may be difficult to meet in a densely populated board and the large and heavy heat sinks expose the attached component to shock and vibration problems during handling and shipping, especially with surface mount components (i.e., components electrically connected to a circuit board via solder balls, or the like). Consequently, the clutter of heatsinks, fastening mechanisms, and adhesives often results in a board with inadequate cooling means and unreliable electrical connections. Manufacturing, troubleshooting, and reworking such a board is difficult, and in some cases, practically even impossible.
Therefore, there is a need for an efficient and cost-effective heat sink apparatus that accommodates a circuit board having a multilevel surface with high power components in close proximity. It is also desirable that the apparatus ensure high structural integrity and reliable electrical connections for a heavy complex assembly. Further, the apparatus should simplify manufacturing, rework, and troubleshooting and use conventional cooling devices (e.g., tube axial fans and heatsinks). Finally, it is desirable that the apparatus allow for reuse of the heat sink, circuit board, and components thereon after rework and troubleshooting.